The present invention relates to organic light-emitting diode devices and methods for making such devices, which use an inorganic buffer structure and a sputtered metal or metal alloy layer on such inorganic buffer structure.
Organic electroluminescent (OEL) device, alternately known as organic light emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency; it is operable with a low driving voltage of the order of a few volts and viewable from oblique angles. These unique attributes are derived from a basic OLED structure comprising of a multilayer stack of thin films of small-molecule organic materials sandwiched between an anode and a cathode. Tang et al in commonly-assigned U.S. Pat. Nos. 4,769,292 and 4,885,211 have disclosed such a structure. The common electroluminescent (EL) medium is comprised of a bilayer structure of a hole-transport (HTL) layer and an electron-transport layer (ETL), typically of the order of a few tens of nanometer (nm) thick for each layer. The anode material is usually an optically transparent indium tin oxide (ITO) film on glass, which also serves as the substrate for the OLED. The cathode is typically a reflective thin film. Selection of electrode materials is based on work functions. ITO is most commonly used as the anode because it has a high work function. Mg:Ag alloys are generally used as electron-injecting contacts because they have lower work functions. Lithium containing alloys such as Al:Li, Ag:Li and LiF/Al contacts also provide efficient electron injection. The device emits visible light in response to a potential difference applied across the EL medium. When an electrical potential difference is applied at the electrodes the injected carriersxe2x80x94hole at the anode and electron at the cathodexe2x80x94migrate towards each other through EL medium and a fraction of them recombine to emit light.
In the fabrication of OLED vapor deposition method is used. Using this method, the organic layers are deposited in thin-film form onto the ITO glass substrates in a vacuum chamber, followed by the deposition of the cathode layer. Among the deposition methods for the cathode, vacuum deposition using resistive heating or electron-beam heating has been found to be most suitable because it does not cause damage to the organic layers. However, it would be highly desirable to avoid these methods for fabrication of the cathode layer. This is because they are inefficient processes. In order to realize low-cost manufacturing one must adopt and develop a proven robust high-throughput process specific to OLED fabrication. Sputtering has been used as a method of choice for thin film deposition in many industries. Conformal, dense, and adherent coatings, short cycle time, low maintenance of coating chamber, efficient use of materials are among few of the benefits of sputtering.
The fabrication of the OLED cathode layer employing high-energy deposition process such as sputtering is not commonly practiced because of the potential damage inflicted on the organic layers, and thus degradation of the OLED performance. Sputter deposition takes place in a complex environment that comprises of energetic neutrals, electrons, positive and negative ions and emissions from the excited states that can degrade the organic layers upon which the cathode is deposited.
Liao et al (Appl. Phys. Lett. 75,1619 [1999]) investigated using x-ray and ultraviolet photoelectron spectroscopies the damages induced on Alq surfaces by 100 eV Ar+irradiation. It is revealed from core level electron density curves that some Nxe2x80x94Al and Cxe2x80x94Oxe2x80x94Al bonds in Alq molecules were broken. The valance band structure is also tremendously changed implying the formation of a metal-like conducting surface. It is suggested that this would cause nonradiative quenching in OLEDs when electrons are injected into the Alq layer from the cathode and also would results in electrical shorts.
During sputter deposition of cathode the Alq surface is subjected to high doses of Ar+ bombardments at several hundred volts. As shown by Hung et al (J. Appl. Phys. 86, 4607 [1999]) that a dose only of 9xc3x971014/cm2 altered the valance band structure. However, sputtering a cathode on Alq surface in Ar atmosphere would degrade the device performance.
Sputtering damage is somewhat controllable, at least to some extent, by properly selecting the deposition parameters. In the European patent applications EP 0 876 086 A2, EP 0 880 305 A1, and EP 0 880 307 A2, Nakaya et al. of TDK Corporation disclose a method of depositing a cathode by a sputtering technique. After depositing all organic layers, with vacuum still kept, the devices was transferred from the evaporation to a sputtering system wherein the cathode layer was deposited directly on the emission layer. The cathode was an Al alloy comprised of 0.1-20 a % Li that additionally contained at least one of Cu, Mg and Zr in small amounts and in some cases had a protective overcoat. The OLED devices thus prepared using no buffer layer were claimed to have good adhesion at the organic layer/electrode interface, low drive voltage, high efficiency and exhibited a slower rate of development of dark spot. Grothe et al. in patent application DE 198 07 370 C1 also disclose a sputtered cathode of an Al:Li alloy which had relatively high Li content and having one or more additional elements chosen from Mn, Pb, Pd, Si, Sn, Zn, Zr, Cu and SiC. In all of those examples no buffer was used, yet electroluminescent was produced at lower voltage. Some sputtering damage was possibly controlled by employing a low deposition rate. It is easily anticipated that by lowering sputtering power the damage inflicted on the organic layers can be reduced. At low power, however, the deposition rate can be impracticably low and the advantages of sputtering are reduced or even neutralized.
To minimize damage during high speed sputtering of cathodes a protective coating on the electron-transport layer can be useful. The protective layer, alternately termed as the buffer layer, must be robust to be effective. However, in addition to being resistant to plasma, the buffer layer must not interfere with the operation of the device and must preserve the device performance. Parthasarathy et al (J. Appl. Phys. 72, 2138 [1998]) reported an application of a buffer layer consisting of copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) during sputtering deposition of a metal free cathode. The buffer layer prevented damage to the underlying organic layers during the sputtering process. Hung et al (J. Appl. Phys. 86, 4607 [1999]) disclosed the application of CuPc buffer layers that permitted high-energy deposition of a cathode. The cathode contained a dopant, e.g. Li, which was believed to diffuse through the buffer layer and provided an electron injecting layer between the organic light emitting structure and the buffer layer. In the patent application EP 0 982 783 A2 Nakaya et al. disclose a cathode of Al:Li alloy. The cathode was prepared by sputtering using a buffer layer constructed of a porphyrin or napthacene compound that was deposed between the emission and the cathode. The device containing the sputtered electrode exhibited low drive voltage, high efficiency and retarted dark spot growth. Although it was claimed in all those references that efficient devices were made, none were said to have eliminated the sputter damage.
The shortcomings of prior art device structures are that they are not ideally suited for devices that contain dopants emitting in different colors. While CuPc is largely transparent in the green region of the wavelength, the transparency in red and blue wavelength length regions is substantially lower. To be useful in full color devices the buffer should have uniformity of transparency in a greater range of wavelength. Another undesirable feature is that the phthalocyanine layer must be about 20 nm thick requiring long deposition time.
It is therefore an object of the present invention to provide an OLED device structure, which has relatively uniform transparency in the visible wavelength range and that, offers significant protection against damage during sputtering deposition of electrode.
The above object was achieved in an OLED device, comprising:
a) a substrate;
b) an anode formed of a conductive material over the substrate;
c) an emissive layer having an electroluminescent material provided over the anode layer;
d) a buffer structure including at least two layers, a first buffer layer provided over the electron-transport layer and containing an alkaline halide and a second buffer layer provided over the first buffer layer and containing a metal or metal alloy and having a work function of between 2.0 to 4.0 eV; and
e) a sputtered layer of a metal or metal alloy provided over the buffer structure.
An advantage of the present invention is that damage to the organic layers during sputtering deposition of cathodes is minimized. The present invention permits high sputtering rates and is suitable for full color large-area devices and displays.
The buffer structure in accordance with the invention having two buffer layers exhibited substantially superior performance in comparison to that of devices having only the metal/metal alloy buffer layer but otherwise identical in structure.
The buffer structure is very thin and has relatively uniform transparency in the visible wavelength range and that offers significant protection against damage during sputtering deposition of cathodes
Another advantage of the present invention is that OLED devices produced by the sputtering deposition method are efficient and operable with a low drive voltage.